Semiconductor device and method for forming the same

ABSTRACT

In a field effect type device having a thin film-like active layer, there is provided a thin film-like semiconductor device including a top side gate electrode on the active layer and a bottom side gate electrode connected to a static potential, the bottom side gate electrode being provided between the active layer and a substrate. The bottom side gate electrode may be electrically connected to only one of a source and a drain of the field effect type device. Also, the production methods therefor are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Div of Ser. No. 08/351,135 Nov. 30, 1994 U.S. Pat.No. 5,917,221 which is a Con of Ser. No. 08/072,127 Jun. 7, 1993 ABN.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an insulation gate type semiconductordevice such as a thin film transistor (TFT) having a thin film activelayer (i.e., an activated region or a channel region) formed on aninsulation substrate. A field to which the invention pertains is asemiconductor integrated circuit, a liquid crystal display device, anoptical reading device or the like.

2. Description of the Prior Art

Recently, researches and developments have been made as to insulationgate type semiconductor devices having thin film active layers oninsulation substrates. In particular, continuous efforts have been madeon so-called thin film transistors (TFTs). These TFTs are intended to beused for controlling respective image elements of matrix structure in adisplay device such as an LCD. Depending upon a material to be used anda crystalline condition of the semiconductors, TFTs are classified intoamorphous silicon TFTs and polycrystal silicon TFTs. However, recently,a material having an intermediate condition between the polycrystallinecondition and amorphous condition has been studied. This is called asemi-amorphous condition and is considered as a condition where smallcrystals are floated on an amorphous formation.

Also, in a single crystal silicon IC, a polycrystal silicon TFT is usedas a so-called SOI technique. For instance, this is used as a loadtransistor in a highly integrated SRAM. However, in this case, anamorphous silicon TFT is hardly used.

In general, an electric field mobility of a semiconductor under theamorphous condition is small, and it is therefore impossible to use thesemiconductor as TFTs which need high speed operation. Also, in theamorphous silicon, the electric field mobility of P-type is small, andit is impossible to produce a P-channel type TFT (TFT of PMOS).Accordingly, it is impossible to form a complementary MOS circuit (CMOS)in combination with N-channel type TFT (TFT of NMOS).

However, TFTs formed of amorphous semiconductors have a feature thattheir OFF current is small. Therefore, such TFTs have been used where anextremely high speed operation is not needed like a liquid crystalactive matrix transistor, one-way conductive type TFTs may besatisfactorily used and TFTs having a high charge holding capacity areneeded.

On the other hand, a polycrystal semiconductor has a larger electricfield mobility than that of an amorphous semiconductor. Therefore, inthis case, it is possible to effect high speed operation. For example,with TFTs using a silicon film recrystallized through a laser annealtechnique, it is possible to obtain a large electric field mobility of300 cm²/Vs. This value is considered very high in view of the fact thatthe electric field mobility of a regular MOS transistor formed on asingle crystal silicon substrate is approximately 500 cm²/Vs. Inaddition, the operation speed of the MOS circuit on the single crystalsilicon substrate is considerably limited by an inherent capacitancebetween the substrate and wirings. In contrast, since the TFT is locatedon the insulation substrate, such a limitation is no longer needed and aconsiderably high speed operation is expected.

Also, it is possible to obtain PTFTs as well as NTFTs from polycrystalsilicon, and hence it is possible to form a CMOS circuit thereby. Forexample, in a liquid crystal display device, a so-called monolithicstructure is known in which not only active matrix portions but alsoperipheral circuits (such as drivers or the like) are formed bypolycrystal CMOS TFTs. This point is noticed also in the TFTs used inthe aforesaid SRAMs. In this case, PMOSs are formed by TFTs and are usedas a load transistor.

However, in general, the polycrystal TFTs have an increased leak currentand a poor performance of holding the electric charge of image elementsof the active matrix since the electric field mobility of thepolycrystal TFTs is larger than that of amorphous TFTs. For example, inthe case where the polycrystal TFTs are used as the liquid crystaldisplay elements, since conventionally, the size of the image elementsis several hundreds of micrometers square and the image elementcapacities are large, there have been no serious problems. However,recently, the fine image elements have been used in accordance with ahigh resolution, and the image element capacities become small. Theconventional image elements would be insufficient for stable staticdisplay.

There have been several solutions for the current leakage problemsinherent in such polycrystal TFTs. One of the methods is to thin anactive layer. It is reported that the OFF current would be small by themethod. For instance, it is known that a thickness of the active layeris 25 nm whereby the OFF current might be less than 10⁻¹³A. It would behowever very difficult to crystallize a thin semiconductor film and itis actually known that the thin semiconductor film could not easily becrystallized.

The method in which the active layer is thinned leads to the phenomenonin which a source/drain region is thinned. This is because thesemiconductor film is formed so that the source/drain region is producedsimultaneously with the formation of the active layer in accordance witha conventional production method and the source/drain region and theactive layer have the same thickness. This would also lead to theincreased resistance of the source/drain region.

For this reason, a method is used in which a thickness of almost all thesource/drain region is increased. This means that a mask process isadditionally used. This is undesired from the view point of productiveyield.

Also, according to the present inventors' knowledge, in the TFTs where athickness of the active layer is 50 nm or less, a MOS threshold voltageis largely shifted, and this phenomenon is remarkable in case of NMOS's.The threshold voltage would be zero or negative values. If, thus, theCMOS is formed by the TFTs, the operation would be unstable.

On the other hand, if the thickness of the active layer would beincreased, the leakage current would be increased. The magnitude thereofis not in proportion to the thickness of the active layer. It istherefore reasonable that the leakage current would be increased in anon-linear manner due to some causes. The present inventors have studiesand found that almost all the leakage current of the TFTs where theactive layer is thick may flow through a part of the active layer on thesubstrate side in a bypass fashion. Two causes thereof might be foundout. One cause is that there is a charge fixed to an interface energeticposition between the substrate and the active layer. The other cause isthat movable ions such as sodium or the like enter from the substrateinto the active layers to thereby make conductive the part of the activelayer on the substrate side. The later cause may be overcome byincreasing a performance of the cleaning process.

However, whatever the interface between the substrate and the activelayer was made clean, it was impossible to overcome the problem of theformer cause. For example, the direct formation of the active layer onthe substrate would lead to raising the interface energetic position.Accordingly, it was impossible to obviate the problem of the leakagecurrent even if an oxide layer (such as heat oxide film of silicon)having a high quality to the same extent as that of the gate oxide filmwas used as an underlayer and the active layer was formed thereon.Namely, it has been found that it is difficult to remove the fixedcharge.

SUMMARY OF THE INVENTION

In order to solve the above-noted defects or difficulties, according tothe present invention, an additional gate electrode (hereinafterreferred to as a bottom side gate electrode) is formed between asubstrate and an active layer, and this gate electrode is kept at asuitable potential whereby the stationary charge described above may becancelled.

According to the present invention, in a field effect type device havinga thin film-like active layer, there is provided a thin film-likesemiconductor device comprising a top side gate electrode on the activelayer and a bottom side gate electrode connected to a static potential,the bottom side gate electrode being provided between the active layerand a substrate.

According to another aspect of the invention, in a field effect typedevice having a thin film-like active layer, there is provided a thinfilm-like semiconductor device comprising a top side gate electrode onthe active layer and a bottom side gate electrode (rear electrode)electrically connected to only one of a source and a drain of the fieldeffect type device, the bottom side gate electrode being providedbetween the active layer and a substrate.

According to still another aspect of the invention, there is provided athin film-like semiconductor device comprising a bottom side gateelectrode (rear electrode) on a substrate having an insulating surface,a semiconductor layer having N-type and P-type impurity regions forcovering the bottom side gate electrode, and two gate electrodesprovided on the semiconductor layer, one of the last-mentioned gateelectrodes being located out of the bottom side gate electrode. A p-typetransistor is provided on the insulating surface and comprises an activeregion and a gate electrode provided on the active region. An n-typetransistor is provided on the insulating surface and comprises anotheractive region and another gate electrode provided on the another activeregion. The active region of only one of the p-type transistor and then-type transistor is provided on the rear electrode. The rear electrodeis kept at a potential of the source of the only one of the p-typetransistor and the n-type transistor.

Preferably, the gate electrode of P-channel type transistor is locatedout of the bottom side gate electrode.

According to the invention, there is provided a method for producing athin film-like semiconductor device, comprising the following steps:selectively forming a first semiconductor coating film, having a firstconductive (conductivity) type, on a substrate having an insulatingsurface; forming a first insulating coating film on the firstsemiconductor coating film; forming a second semiconductor coating filmfor covering the first insulating coating film; forming a secondinsulating coating film on the second semiconductor coating film;forming at least two gate electrode portions on the second insulatingcoating film; dispersing impurities for the first conductive type intothe second semiconductor coating film in a self-alignment mannerrelative to the gate electrode portions; and after the dispersing step,in a self-alignment manner relative to at least one of the gateelectrode portions, dispersing impurities for a conductive(conductivity) type opposite the first conductive type in the secondsemiconductor coating film below which the first semiconductor coatingfilm is not present.

According to the invention, there is provided a method for producing athin film-like semiconductor device, comprising the following steps:forming, on a substrate having an insulating surface, a first conductivecoating layer made of one selected from the group essentially consistingof semiconductor and metal; forming a first insulating coating film onthe first conductive coating film; forming a first semiconductor coatingfilm on the first insulating coating film; forming a second insulatingcoating film on the first semiconductor coating film; forming an etchingmask material on the second insulating coating film; forming a hole inthe etching mask material; forming a contact hole in the secondinsulating coating film in accordance with an isotropic etching processwhile using the etching mask material as a mask, that is, through anopening of the etching mask; forming a hole (an opening) in the firstsemiconductor coating film in accordance with an anisotropic etchingprocess while using the etching mask material as a mask; and forming ahole (an opening) in the first insulating coating film in accordancewith one of the isotropic etching process and the anisotropic etchingprocess while using the etching mask material as a mask, thereby formingan electrode connected between the first conductive coating film and thefirst semiconductor film.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are illustrations of inventive concept of TFTs accordingto the invention;

FIGS. 2A and 2B are cross-sectional views showing examples of TFTs;

FIGS. 3A to 3H are illustrations showing the operation of the TFTsaccording to the invention;

FIG. 4 shows the operation of the TFTs according to the prior art;

FIGS. 5A to 5F show the steps for producing the TFTs according to thepresent invention;

FIGS. 6A to 6F show the application of the TFTs according to theinvention; and

FIGS. 7A to 7E show the process for manufacturing the TFTs according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to theaccompanying drawings.

FIGS. 1A and 1B show an inventive concept of the present invention.Reference character A denotes a gate electrode which is well known inthe art. Reference character B denotes a gate electrode provided on abottom side. Such a bottom side gate electrode B may be disposed so asto be overlapped with the face surfaces of source/drain regions as shownin FIG. 1A. However, in this arrangement, additional capacitancesbetween the source/drain regions and the bottom side gate electrodewould be increased. In the case where the high speed operation or thelike is required, as shown in FIG. 1B, it is possible to take thearrangement where the bottom side gate electrode B is located so as notto overlap with either one or both of the source region and the drainregion. In any case, it is important that the bottom side gate electrodeis overlapped with at least a part of the active layer. In order toinsure the advantage of the invention, it is located so as to transversethe active layer as much as possible.

For instance, in a conventional NMOS, in the case where the electricpotentials of the source and the gate are kept at zero and the potentialof the drain is kept at 10V, the drain current have to be zero in anideal condition. However, the fixed charge on the substrate cause theactive layer to be kept in a weak inversion state. Therefore, the draincurrent will flow by a thermal excitation. This is shown in FIG. 4.Namely, in a conventional TFT, a weak inversion region is formed by thefixed charge on the substrate side as shown in FIG. 4. Since the fixedcharge is present without any change whatever voltage is applied to thegate electrode, it becomes a source of the leakage current. However, inthe case where a thickness of the active layer is extremely decreased,the affect of the gate electrode is applied also to the substrate sothat this weak reversed region will be obviated by the potential of thegate. It is assumed that various reports that the leakage current may bereduced by thinning the active layer without any good reason may bebased upon the foregoing reasons. However, in the models shown in FIG.4, it has been found that the threshold voltage is readily shifted, andthe conventional method is not an essential solution.

The purpose of the present invention is to remove the affect of thefixed charge by providing the above-described bottom side gate electrodeand keeping the potential of the bottom side gate electrode (rearelectrode) at zero or negative values. FIGS. 2A and 2B show examples ofthe present invention in which a bottom side gate electrode (a rearelectrode) is electrically connected to a source region through acontact hole provided in a portion of an insulating film so that thebottom side gate electrode may be always kept at the same potential asthat of the source. In FIG. 2A, the bottom side gate electrode 9 isoverlapped exactly with the source region 6 and the drain region 5. Inthis case, the manufacture process would be relatively simple and yieldis high since no stepped portion would be formed in the gate electrode9.

In order to produce an element having such an arrangement, the followingsteps should be carried out. Namely, a coating film to be the bottomside gate electrode 9 and an insulating film 8 are formed on thesubstrate. A contact hole 10 is formed in the insulating film 8 and asemiconductor layer is formed therein. These components are subjected toa patterning process. Then, the gate insulating film 4 and the gateelectrode 1 are formed and the drain region 5 and the source region 6are formed in a self-alignment manner. Parts where no impurities aredoped will become the active layer 7. Finally, a drain electrode 2 and asource electrode 3 are formed thereon. The number of the masks used inthe foregoing steps is four (five in the case where the source electrode3 and the drain electrode 2 are not simultaneously formed).

On the other hand, FIG. 2B shows the example where the bottom side gateelectrode 19 is not overlapped with the drain region 15. The step of thebottom side gate electrode causes an adverse affect to be applied to thegate electrode 11. For this reason, there would be a fear that theexfoliation or removal problem would be applied to the gate electrode.Also, the number of processing steps is increased in comparison with thecase shown in FIG. 2A. Namely, first of all, the bottom side gateelectrode 19 is patterned, and subsequently, the insulating film 18 isformed to form a contact hole 20. Then, the semiconductor layer isformed and is patterned. Then, the gate electrode 11 is patterned. Thesource region 14, drain region 15 and active region 17 are formed in aself-alignment manner. The source electrode 13 and the drain electrode12 are formed thereon. The number of the masks used in the foregoingsteps is five or six. It is an ideal condition that the additionalcapacitance is reduced and the bottom side electrode is formed in theself-alignment manner with the source region and the drain region inorder to simplify the process.

The material for the bottom side gate electrode 9, 19 should be selectedin view of the process to be applied to the material. For instance, inthe case where the gate insulation film is formed in accordance with thethermal oxidation method, the material should stand the high temperaturecorresponding to the method and the dispersion of the different harmfulelements from the bottom side gate material to the active layer shouldbe avoided. For example, if the active layer is formed of silicon andthe gate insulation film is a thermal oxidation film of silicon, ingeneral, the maximum processing temperature exceeds 1,000° C.Accordingly, a doped polysilicon is desired as the material for thebottom side gate electrode.

Also, in a low temperature process in which the maximum processingtemperature is about 600° C., it is possible to use the doped siliconbut it is more preferable to use lower resistance substances such aschrome, tantalum and tungsten. Of course, any other material may be usedas a design choice as desired.

FIGS. 3A to 3H show the operation of the thus constructed TFT. FIGS. 3Ato 3H show the case of an NMOS. However, in case of a PMOS, theinequalities used in these figures should be directed opposite thoseshown. First of all, the explanation will be made as to the case wherethe gate potential V_(G) is equal to either lower one of the sourcepotential V_(S) or the drain potential V_(D). In this case, as shown inFIG. 4, since the potentials of the source and the drain are notsymmetric with each other, the state depends upon the magnitude of thepotential V_(D). When the relation, V_(S)<V_(D), is established, asshown in FIG. 3A, the gate electrode, the bottom side gate electrode andthe source are kept at the same potential so that the electrons aredischarged from these regions to form depletion regions or accumulationregion. Inversely, when the relation, V_(S)<V_(D), is established, asshown in FIG. 3B, the gate electrode side is a depletion region but aninversion region is formed on side of the bottom side gate electrode toallow the drain current to flow. The above discussion is very rough andmore strictly, the threshold voltage should be considered but thediscussion would be used to understand the concept of the presentinvention.

Under the condition, V_(D)>V_(S) and V_(G)<V_(S), is given, thedepletion region expands over all the active layer (see FIG. 3C),whereas under the condition, V_(D)>V_(S) and V_(G)<V_(S), is given, theinversion region is formed on the gate electrode side (see FIG. 3D).Also, under the conditions, V_(D)<V_(S) and V_(G)<V_(D), the inversionregion is formed on the bottom gate electrode side to allow the draincurrent to flow (see FIG. 3E), whereas under the conditions, V_(D)<V_(S)and V_(G)>V_(D), the inversion regions are formed on both sides (seeFIG. 3F).

The state will become more complicated in the case where V_(D) is equalto or substantially equal to V_(S). Namely, in this case, since thereare no lines of electric force flowing from the source to the drain (orfrom the drain to the source), the affect of the fixed charge on thebottom gate electrode side causes a weak inversion region to be formedto generate the leakage current as in the conventional TFTs (see FIGS.3G and 3H).

It is practically convenient that the bottom side gate electrode is keptat the same potential as that of the source or the drain. If it isimpossible to meet this requirement, it is sufficient to keep the bottomgate electrode at the same potential as that of other power supply(other power source). Also, even if it is kept at the same potential asthat of the source or drain, if the potential is kept unchanged, thereis little adverse affect to the operation characteristics of theelement.

For example, in the case where the amount of leakage in the OFFcondition is reduced, and the ON/OFF operation is carried out by theTFT, the potentials are selected so as to realize the states shown inFIG. 3A or 3C (OFF condition) and FIG. 3D or 3F or FIG. 3H (ONcondition). Also, it is possible to use the element to form a CMOSinverter circuit.

The problem of the fixed charge is remarkable mainly in the NMOS.Therefore, the PMOS is made in the same manner as in the conventionalmethod and the present invention may be applied only to the NMOS.However, in the case where the charge is negative, the charge causes aproblem even in the PMOS and hence it is preferable to apply theinvention for both cases.

EXAMPLE 1

A method for producing crystallized silicon TFTs through a hightemperature process according to the present invention will now bedescribed. In this example, the gate electrode as well as the bottomside gate electrode was made of doped polysilicon. The manufacturingprocess is well known in the art, i.e., conventional processingtechniques for various semiconductor integrated circuits and hence thedetailed discussion thereof will be omitted.

A polycrystal silicon film which was doped with phosphorus of 10¹⁹ to5×10²⁰ cm⁻³, for example, 8×10¹⁹ cm⁻³ was formed on a quartz substrate21 with a thickness of 100 to 500 nm, for example, 200 nm according to alow pressure CVD process. This was thermally oxidized in an oxygenatmosphere kept at 1,000° C. to form a silicon coating film 22 andsilicon oxide film 23. A thickness of the silicon oxide was in the rangeof 50 to 200 nm, preferably at 70 nm. In this case, a silicon film whichis doped with no impurity may be formed and then the impurity may bedoped into the silicon film or otherwise the impurity may be dopedthereinto after the silicon film has been thermally oxidized.

Thereafter, the amorphous silicon film 24 which was not doped withimpurities was accumulated to have a thickness of 100 to 1,000 nm, forexample, 300 nm. During the accumulation, a temperature of the substratewas kept in the range of 450 to 500° C., for example, 480° C. Also, thematerial gas was monosilane or polysilane (disilane, trisilane).However, disilane was stabler than polysilanes over the trisilane andmight cause the better film to be formed than the monosilane. Thecrystal had been slowly grown at 600° C. for twelve hours. Thearrangement until this step is shown in FIG. 5A.

Subsequently, the patterning was effected so that island-likesemiconductor regions (i.e., silicon islands) and the rear electrodes(the bottom side gate electrodes) thereunder were formed. A siliconoxide film 25 which was to be a gate insulating film was formed to havea thickness 50 to 500 nm, for example, 150 nm by thermal oxidation inthe oxygen atmosphere. This condition is shown in FIG. 5B.

Further, a polycrystal silicon film doped with phosphorus was formed toa thickness in the range of 300 to 1,000 nm, for example, 500 nmaccording to the low pressure CVD method, and the film was subjected tothe patterning technique to form the gate electrode 26. An ion injectionwas effected in a self-alignment manner by using the gate electrode as amask and was annealed at 1,000° C. to form the source region 28 and thedrain region 27. An active region (a channel) was then formed in theisland-like semiconductor region between the source region 28 and thedrain region 27. An insulating material 29 was formed in accordance witha plasma CVD method of TEOS and a contact hole was provided to theinsulating material to form the drain electrode 30. This state is shownin FIG. 5C.

Thereafter, the source electrode was formed. This process was peculiar.Thus, the process will be described in detail. After the drain electrodehad been formed, an insulating material 31 to be interposed betweenlayers was formed. A photoresist 32 was formed by a spin coatingprocess. A hole 33 was formed for forming a contact hole of the sourceelectrode.

Subsequently, the intermediate insulating layer and the gate insulatingcoating (both made of silicon oxide) were etched by an isotropic etchingtechnique such as an isotropic dry etching process or an isotropic wetetching process. In this case, it is desired to selectively and solelyetch the silicon oxide coating. For example, it is preferable to use ahydrofluoric acid as an etchant. In a relatively long period of etchingtime, the etching expanded to side walls of the contact hole. Thecontact hole 34 which was larger than the hole 33 was formed. This stateis shown in FIG. 5D.

Then, an anisotropic etching process such as an RIE (reactive ionetching) was effected so that the source region 28 was etchedsubstantially corresponding to the hole 33 to form a contact hole 35.This state is shown in FIG. 5E. Thereafter, a thin silicon oxide layerpresent between the source region and the bottom side gate electrode wasremoved.

After the photoresist had been removed, the source electrode 36 wasformed of metal wiring material. Namely, by the above-describedtwo-stage etching process, a sufficient contact of a sufficient contacthole was made between the source region and the bottom side gateelectrode. This is shown in FIG. 5F. Thus, the TFT was completed. Asshown in FIG. 5F, an insulating film exists between the rear electrodeand the active region.

A CMOS inverter circuit was constituted by combining the TFTs of thethus formed NMOS and PMOS as shown in FIG. 6A. A circuit diagram of thecircuit is shown in FIG. 6B. In this inverter circuit, the bottom sidegate electrode is always kept at a potential of the source (V_(H) incase of the PMOS and V_(L) in case of the NMOS). Namely, in a staticcondition, if Vin is V_(H) (i.e., Vout is V_(L)), the NMOS was in thecondition shown in FIG. 3H and the PMOS was in the condition shown inFIG. 3A. Inversely, if Vin is V_(L) (i.e., Vout is V_(H)), the NMOS isin the state shown in FIG. 3A and the PMOS is in the state shown in FIG.3H, thereby extremely suppressing the leak current on the substrateside.

The reason why the leak current may be reduced only by keeping thebottom side gate electrode at the same potential as that of the sourcewill be explained hereunder.

Namely, assume that the drain 61 is higher in potential than the source63 in the NMOS as shown in FIG. 6C. If there would be no bottom sidegate electrode or even if there would be the bottom side gate electrodebut if the bottom side gate electrode 64 would be in a floating state,lines of electric forces from the drain to the source transverses theactive region 62 straightforwardly as shown in FIG. 6C. However, if thebottom side gate electrode is kept at the same potential as that of thesource, the part of the electric force lines which would inherently bedirected to the source is attracted toward the bottom side gateelectrode and is curved as shown in FIG. 6D.

As a matter of fact, since a fixed charge is present on an interfacebetween the active layer region and the insulating coating, the state iscomplicated. Namely, if there would be no bottom side gate electrode orit would be in the floating state, the electric force lines would beaffected by the fixed charge (whose polarity is positive) so thatelectric force lines having a component directed from the insulatingcoating (or the bottom side gate electrode) to the active layer aregenerated. Since the pattern of the electric force lines means that thepotential of the insulating film (or the bottom side gate electrode) ishigher than that of the inner side of the active layer, the electronswill be attracted by the potential so that a weak inversion region isformed close to the insulating film interface. Since this weak inversionregion is continuously generated from the drain to the source, it causesthe leak current.

On the other hand, in the case where the bottom side gate electrode iskept at the same potential as that of the source, even if the stationarycharge is present between the active layer and the insulating film (orthe bottom side gate electrode), since the electric force lines emittedfrom the drain have a component toward the bottom side gate electrode,both the electric force lines are cancelled by each other so that almostno electric force lines are generated from the bottom side gateelectrode to the active layer surface. Also, even if the electric forcelines having such a component are partially generated, since theelectric force lines are not generated over all the region between thesource to the drain, there is almost no fear that the leak current willbe generated.

Thus, by keeping the bottom side gate electrode at the source potential,it is possible to considerably reduce the leak current. For example, inthe case of the CMOS circuit, the maintenance current in the staticcondition is kept substantially at a sum of the leak currents of theNMOS and the PMOS. However, in the conventional TFTs, if the drainvoltage is 5V, approximately 1 pA will flow. For example, in a staticRAM of 1 Mbits, there are about two millions of CMOS inverter circuits,and in order to keep the memory, a current of about 2 micron A willalways flow.

However, according to the present invention, in particular, the leakcurrent was considerably reduced, and the maintenance current for oneCMOS inverter was reduced to 0.01 to 0.1 pA. Thus, the holding currentfor 1 Mbit SRAM was reduced to 0.02 to 0.2 micron A. In the case wherethe present invention is applied to a non-volatile memory provided aback-up battery for an SRAM, it is possible to extend a service life ofthe battery 10 to 100 times longer than that of the conventional one.

It should be noted that there are inherent capacitances C₂ and C₃ of thedrain and the source through the bottom side gate electrode in additionto the capacitance C₁ of the gate electrode and the channel whichcapacitance is incorporated as a design factor in the conventional CMOSinverter circuit. The inherent capacitances serve as loads to reduce thesignal transmission speed during the operation of the inverter and toincrease the consumption power. According to the simple calculation, thesignal delay time is in proportion to the sum of C₂ and C₃ and theconsumption power is in proportion to a four order exponential value ofthe sum.

Accordingly, it is desired to reduce the inherent capacitances as muchas possible. Actually, since the stationary charge is almost positive,it does not adversely affect the PMOS. Accordingly, it is effective touse the PMOS having the same structure as that of conventional ones andto apply the bottom side gate electrode according to the presentinvention only to the NMOS. In a simple consideration, it is possible toreduce the inherent capacitances to half the ones comprising C₂ and C₃,and accordingly to reduce the power loss due to the inherentcapacitances to one sixteenth of the level.

EXAMPLE 2

A method for producing a crystallized silicon TFTs according to a hightemperature process utilizing the present invention will be described inthe following example. In this example, both the gate electrode and thebottom side gate electrode were formed of doped polysilicon. Themanufacture technique is well known as a process for varioussemiconductor integrated circuits and hence detailed explanation will beomitted.

A polycrystal silicon film doped with phosphorus (n-type impurity) underthe same conditions as those of Example 1 was formed on the quartzsubstrate (insulating substrate) 71 and a patterning process was appliedthereto to form a bottom side gate electrode 72. The film was thermallyoxidized in an oxygen atmosphere to form a silicon oxide film 73.Thereafter, under the same conditions as those of Example 1, anamorphous silicon film 74 which had not been doped with the impuritieswas accumulated thereon and the crystallization was grown by a heatannealing process. This state is shown in FIG. 7A.

Subsequently, the patterning process was effected on the film to form anisland-like semiconductor regions (silicon islands) and a thermaloxidized film 75 was formed in the same manner as in Example 1.Furthermore, a gate electrode 77 for an NMOS and a gate electrode 76 fora PMOS were formed by the doped silicon, and N-type impurity ions wereinjected into the island-like semiconductor region in a self-alignmentmanner to form an impurity region 78. In this case, although N-typeimpurities (for example, phosphorus or arsenic) were injected into thebottom side gate electrode, there was no problem because the bottom sidegate electrode itself is of an N-type. This state is shown in FIG. 7B.

Then, a part on the right side of the shown TFT was covered byphotoresist or the like, and the P-type impurity ions (boron or thelike) were injected into a portion of the silicon film 74 which is notprovided on the bottom side gate electrode 72. Through these steps, thesource 79 and the drain 80 of the PMOS and the source 82 and the drain81 of the NMOS were produced. This state was shown in FIG. 7C.

Thereafter, the photoresist 84 was applied over all the surface of thearticle. Holes 85 to 87 were formed at positions where contact holeswere to be formed. Then, in the same process as in Example 1, thecontact holes (openings) 88 to 90 were formed in the insulating layersbetween the layers and the gate oxide film (both of which are formed ofsilicon oxide) by the isotropic etching process. In any case, thecontact holes were expanded more than the holes formed in the resist.Furthermore, according to the anisotropic etching process, the siliconlayers were etched to the holes 85 to 87 and with respect to the contacthole 90, the thin silicon oxide layer below the contact hole 90 was alsoetched. The state is shown in FIG. 7D.

Finally, the electrodes 91 to 93 were formed of metal material asconductive regions. This state is shown in FIG. 7E. As shown in FIG. 7E,the rear electrode is electrically connected with the source of then-type transistor. The electrode 91 was formed for a high potential, theelectrode 93 was formed for a low potential, and the electrode 92 wasformed for an output terminal to form an inverter. There is a fear thatthe inverter thus produced would have a large leak of the PMOS incomparison with that according to Example 1. However, in general, theleak current of the NMOS according to the present invention was reducedby one or two digits, whereas the leak current of the PMOS was improvedby about one digit or less. As a result, even if the present inventionwas applied only to the NMOS, the difference in leak current between theNMOS and PMOS was reduced. Accordingly, the degradation ofcharacteristics of the CMOS inverter circuit was not particularlyobserved.

In the CMOS inverter, under the high voltage input condition (where theNMOS was turned on and the PMOS was turned off), the leakage currentdepended upon the leakage current of the PMOS, whereas under the lowvoltage input condition (where the NMOS was turned off and the PMOS wasturned on), the leakage current depended upon the leakage current of theNMOS. In the conventional TFTs, the leakage current of the NMOS wasgreater 100 times or more than that of the PMOS, and when this wasapplied to the SRAM circuit, in a single memory cell, any inverter wasin the low voltage input condition (where the NMOS was turned off andthe PMOS was turned on). After all, the leakage current of the SRAMcircuit depended upon the leakage current of the NMOS.

Accordingly, in a practical aspect, as in this example, it wassufficient to reduce the leak current of the NMOS by one to two digitsby providing the bottom side gate electrode only onto the NMOS. If thebottom side gate electrode would be provided for both the NMOS and thePMOS, almost all the leakage current would depend upon the NMOS. Rather,in consideration of the demerit due to the inherent capacitances of thebottom side gate electrode and the drain, it is reasonable to provide nobottom side gate electrode onto the PMOS.

As described above, it was possible to produce the TFTs having excellentcharacteristics with little leakage current. Also, as shown above, itwas possible to enhance the characteristics of the CMOS by combining theTFTs. The TFTs may be applied to the high speed memory and the highspeed logic circuit as well as liquid crystal displays and imagesensors. The present invention may be applied to these equipments, andin addition, it is possible to enhance the various characteristics suchas reliability and power consumption of these devices. In the specificexamples, the high temperature process was taken into consideration andthe specific method for being applied thereto was discussed. It isapparent that the present invention may be applied to the lowtemperature process. Incidentally, in the case where the low temperatureprocess is used, an anode oxidation process as shown in Japanese PatentApplication Laid-Open Nos HEI 4-38637 and HEI 4-54322 which are by thepresent applicants may be effectively utilized.

Also, the TFTs are used in a conventional monocrystal integratedcircuit. However, apparently, it is possible to use the TFTs accordingto the present invention instead of the regular MOS transistors tofurther enhance the characteristics of the circuit rather than theconventional auxiliary purpose. Thus, the industrial evaluation of thepresent invention is large.

What is claimed is:
 1. A semiconductor device comprising: at least ap-channel thin film transistor and an n-channel thin film transistorprovided over a substrate having an insulating surface, wherein each ofsaid p-channel thin film transistor and said n-channel thin filmtransistor comprises a pair of impurity regions, a channel regionprovided between said impurity regions, and a gate electrode providedover said channel region with a first gate insulating film therebetween,wherein only said n-channel thin film transistor further comprises aconductive layer below said channel region with a second insulating filmtherebetween, and wherein said conductive layer is overlapped with oneof said pair of impurity regions of said n-channel thin film transistorand said conductive layer is not overlapped with the other of said pairof impurity regions of said n-channel thin film transistor.
 2. Thedevice of claim 1 wherein said conductive layer comprises a materialselected from the group consisting of a doped polysilicon, chromium,tantalum and tungsten.
 3. The device of claim 1 wherein said substrateis a quartz.
 4. The device of claim 1 wherein said semiconductor deviceis one of a liquid crystal display device, image sensor, logic circuitand a memory.
 5. The device of claim 1 wherein said channel region isformed of crystalline silicon.
 6. A semiconductor device comprising: atleast a p-channel thin film transistor and an n-channel thin filmtransistor provided over a substrate having an insulating surface,wherein each of said p-channel thin film transistor and said n-channelthin film transistor comprises a pair of impurity regions, a channelregion provided between said impurity regions, and a gate electrodeadjacent to one side of said channel region, wherein only said n-channelthin film transistor further comprises a conductive layer adjacent tothe other side of said channel region opposed to said gate electrode,and wherein said conductive layer is overlapped with one of said pair ofimpurity regions of said n-channel thin film transistor and saidconductive layer is not overlapped with the other of said pair ofimpurity regions of said n-channel thin film transistor.
 7. The deviceof claim 6 wherein said conductive layer comprises a material selectedfrom the group consisting of a doped polysilicon, chromium, tantalum andtungsten.
 8. The device of claim 6 wherein said substrate is a quartz.9. The device of claim 6 wherein said semiconductor device is one of aliquid crystal display device, image sensor, logic circuit and a memory.10. The device of claim 6 wherein said channel region is formed ofcrystalline silicon.
 11. A thin film transistor comprising: a substratehaving an insulating surface; a semiconductor layer formed over saidinsulating surface; a pair of impurity regions formed in saidsemiconductor layer; a channel region in said semiconductor layerbetween said impurity regions; a gate electrode adjacent to one side ofsaid channel region; and a conductive layer adjacent to the other sideof said channel region opposed to said gate electrode, wherein saidconductive layer is overlapped with one of said pair of impurity regionsand said conductive layer is not overlapped with the other of said pairof impurity regions, wherein said conductive layer is overlapped with afirst portion of said channel region so that a second portion of saidchannel region is offset from said conductive layer while said gateelectrode is overlapped with an entire portion of said channel region,and wherein said conductive layer is grounded.
 12. The device of claim11 wherein said conductive layer comprises a material selected from thegroup consisting of a doped polysilicon, chromium, tantalum andtungsten.
 13. The device of claim 11 wherein said substrate is a quartz.14. The device of claim 11 wherein said channel region is formed ofcrystalline silicon.
 15. The device of claim 11 wherein said gateelectrode is formed over said semiconductor layer with an insulatingfilm interposed therebetween.
 16. A thin film transistor comprising: asubstrate having an insulating surface; a semiconductor layer formedover said insulating surface; a pair of impurity regions formed in saidsemiconductor layer; a channel region in said semiconductor layerbetween said impurity regions; a gate electrode adjacent to one side ofsaid channel region; and a conductive layer adjacent to the other sideof said channel region opposed to said gate electrode, wherein saidconductive layer is overlapped with one of said pair of impurity regionsand said conductive layer is not overlapped with the other of said pairof impurity regions, wherein said conductive layer is overlapped with afirst portion of said channel region so that a second portion of saidchannel region is offset from said conductive layer while said gateelectrode is overlapped with an entire portion of said channel region,and wherein said conductive layer and said one of said pair of impurityregions are electrically connected.
 17. The device of claim 16 whereinsaid conductive layer comprises a material selected from the groupconsisting of a doped polysilicon, chromium, tantalum and tungsten. 18.The device of claim 16 wherein said substrate is a quartz.
 19. Thedevice of claim 16 wherein said channel region is formed of crystallinesilicon.
 20. The device of claim 16 wherein said gate electrode isformed over said semiconductor layer with an insulating film interposedtherebetween.
 21. A thin film transistor comprising: a substrate havingan insulating surface; a semiconductor layer formed over said insulatingsurface; a source region and a drain region formed in said semiconductorlayer; a channel region in said semiconductor layer between said sourceregion and said drain region; a gate electrode adjacent to one side ofsaid channel region; and a conductive layer adjacent to the other sideof said channel region opposed to said gate electrode, wherein saidconductive layer is overlapped with said source region and saidconductive layer is not overlapped with said drain region, wherein saidconductive layer is overlapped with a first portion of said channelregion so that a second portion of said channel region is offset fromsaid conductive layer while said gate electrode is overlapped with anentire portion of said channel region, and wherein said conductive layeris grounded.
 22. The device of claim 21 wherein said conductive layercomprises a material selected from the group consisting of a dopedpolysilicon, chromium, tantalum and tungsten.
 23. The device of claim 21wherein said substrate is a quartz.
 24. The device of claim 21 whereinsaid channel region is formed of crystalline silicon.
 25. The device ofclaim 21 wherein said gate electrode is formed over said semiconductorlayer with an insulating film interposed therebetween.
 26. A thin filmtransistor comprising: a substrate having an insulating surface; asemiconductor layer formed over said insulating surface; a source regionand a drain region formed in said semiconductor layer; a channel regionin said semiconductor layer between said source region and said drainregion; a gate electrode adjacent to one side of said channel region;and a conductive layer adjacent to the other side of said channel regionopposed to said gate electrode, wherein said conductive layer isoverlapped with said source region and said conductive layer is notoverlapped with said drain region, wherein said conductive layer isoverlapped with a first portion of said channel region so that a secondportion of said channel region is offset from said conductive layerwhile said gate electrode is overlapped with an entire portion of saidchannel region, and wherein said conductive layer and source region areelectrically connected.
 27. The device of claim 26 wherein saidconductive layer comprises a material selected from the group consistingof a doped polysilicon, chromium, tantalum and tungsten.
 28. The deviceof claim 26 wherein said substrate is a quartz.
 29. The device of claim26 wherein said channel region is formed of crystalline silicon.
 30. Thedevice of claim 26 wherein said gate electrode is formed over saidsemiconductor layer with an insulating film interposed therebetween. 31.A thin film transistor comprising: a substrate having an insulatingsurface; a semiconductor layer formed over said insulating surface; apair of impurity regions formed in said semiconductor layer; a channelregion in said semiconductor layer between said impurity regions; a gateelectrode adjacent to one side of said channel region; and a conductivelayer adjacent to the other side of said channel region opposed to saidgate electrode, wherein said conductive layer is overlapped with one ofsaid pair of impurity regions and said conductive layer is notoverlapped with the other of said pair of impurity regions, wherein saidconductive layer extends beyond a boundary of one of said pair ofimpurity regions and said channel region to overlap said channel regionpartially and said conductive layer does not extend beyond a boundary ofthe other of said pair of impurity regions and said channel region,wherein said gate electrode is overlapped with an entire portion of saidchannel region, and wherein said conductive layer is grounded.
 32. Thedevice of claim 31 wherein said conductive layer comprises a materialselected from the group consisting of a doped polysilicon, chromium,tantalum and tungsten.
 33. The device of claim 31 wherein said substrateis a quartz.
 34. The device of claim 31 wherein said channel region isformed of crystalline silicon.
 35. The device of claim 31 wherein saidgate electrode is formed over said semiconductor layer with aninsulating film interposed therebetween.
 36. A thin film transistorcomprising: a substrate having an insulating surface; a semiconductorlayer formed over said insulating surface; a pair of impurity regionsformed in said semiconductor layer; a channel region in saidsemiconductor layer between said impurity regions; a gate electrodeadjacent to one side of said channel region; and a conductive layeradjacent to the other side of said channel region opposed to said gateelectrode, wherein said conductive layer is overlapped with one of saidpair of impurity regions and said conductive layer is not overlappedwith the other of said pair of impurity regions, wherein said conductivelayer extends beyond a boundary of one of said pair of impurity regionsand said channel region to overlap said channel region partially andsaid conductive layer does not extend beyond a boundary of the other ofsaid pair of impurity regions and said channel region, wherein said gateelectrode is overlapped with an entire portion of said channel region,and wherein said conductive layer and said one of said pair of impurityregions are electrically connected.
 37. The device of claim 36 whereinsaid conductive layer comprises a material selected from the groupconsisting of a doped polysilicon, chromium, tantalum and tungsten. 38.The device of claim 36 wherein said substrate is a quartz.
 39. Thedevice of claim 36 wherein said channel region is formed of crystallinesilicon.
 40. The device of claim 36 wherein said gate electrode isformed over said semiconductor layer with an insulating film interposedtherebetween.
 41. A thin film transistor comprising: a substrate havingan insulating surface; a semiconductor layer formed over said insulatingsurface; a source region and a drain region formed in said semiconductorlayer; a channel region in said semiconductor layer between said sourceregion and said drain region; a gate electrode adjacent to one side ofsaid channel region; and a conductive layer adjacent to the other sideof said channel region opposed to said gate electrode, wherein saidconductive layer is overlapped with said drain region and saidconductive layer is not overlapped with said source region, wherein saidconductive layer is overlapped with a first portion of said channelregion so that a second portion of said channel region is offset fromsaid conductive layer while said gate electrode is overlapped with anentire portion of said channel region, and wherein said conductive layeris grounded.
 42. The device of claim 41 wherein said conductive layercomprises a material selected from the group consisting of a dopedpolysilicon, chromium, tantalum and tungsten.
 43. The device of claim 41wherein said substrate is a quartz.
 44. The device of claim 41 whereinsaid channel region is formed of crystalline silicon.
 45. The device ofclaim 41 wherein said gate electrode is formed over said semiconductorlayer with an insulating film interposed therebetween.
 46. A thin filmtransistor comprising: a substrate having an insulating surface; asemiconductor layer formed over said insulating surface˜a pair ofimpurity regions formed in said semiconductor layer; a channel region insaid semiconductor layer between said impurity regions; a first gateelectrode adjacent to one side of said channel region; and a second gateelectrode adjacent to the other side of said channel region opposed tosaid gate electrode, wherein said second gate electrode is overlappedwith one of said pair of impurity regions and said second gate electrodeis not overlapped with the other of said pair of impurity regions,wherein said second gate electrode is overlapped with a first portion ofsaid channel region so that a second portion of said channel region isoffset from said second gate electrode while said first gate electrodeis overlapped with an entire portion of said channel region, and whereinsaid second gate electrode is grounded.
 47. The device of claim 46wherein said second gate electrode comprises a material selected fromthe group consisting of a doped polysilicon, chromium, tantalum andtungsten.
 48. The device of claim 46 wherein said substrate is a quartz.49. The device of claim 46 wherein said channel region is formed ofcrystalline silicon.
 50. The device of claim 46 wherein said first gateelectrode is formed over said semiconductor layer with an insulatingfilm interposed therebetween.
 51. A thin film transistor comprising: asubstrate having an insulating surface; a semiconductor layer formedover said insulating surface; a pair of impurity regions formed in saidsemiconductor layer; a channel region in said semiconductor layerbetween said impurity regions; a first gate electrode adjacent to oneside of said channel region; and a second gate electrode adjacent to theother side of said channel region opposed to said gate electrode,wherein said second gate electrode is overlapped with one of said pairof impurity regions and said second gate electrode is not overlappedwith the other of said pair of impurity regions, wherein said secondgate electrode is overlapped with a first portion of said channel regionso that a second portion of said channel region is offset from saidsecond gate electrode while said first gate electrode is overlapped withan entire portion of said channel region, and wherein said conductivelayer and said one of said pair of impurity regions are electricallyconnected.
 52. The device of claim 51 wherein said second gate electrodecomprises a material selected from the group consisting of a dopedpolysilicon, chromium, tantalum and tungsten.
 53. The device of claim 51wherein said substrate is a quartz.
 54. The device of claim 51 whereinsaid channel region is formed of crystalline silicon.
 55. The device ofclaim 51 wherein said first gate electrode is formed over saidsemiconductor layer with an insulating film interposed therebetween. 56.A thin film transistor comprising: a substrate having an insulatingsurface; a semiconductor layer formed over said insulating surface; asource region and a drain region formed in said semiconductor layer; achannel region in said semiconductor layer between said source regionand said drain region; a first gate electrode adjacent to one side ofsaid channel region; and a second gate electrode adjacent to the otherside of said channel region opposed to said gate electrode, wherein saidsecond gate electrode is overlapped with said source region and saidsecond gate electrode is not overlapped with said drain region, whereinsaid second gate electrode is overlapped with a first portion of saidchannel region so that a second portion of said channel region is offsetfrom said second gate electrode while said first gate electrode isoverlapped with an entire portion of said channel region, and whereinsaid conductive layer is grounded.
 57. The device of claim 56 whereinsaid second gate electrode comprises a material selected from the groupconsisting of a doped polysilicon, chromium, tantalum and tungsten. 58.The device of claim 56 wherein said substrate is a quartz.
 59. Thedevice of claim 56 wherein said channel region is formed of crystallinesilicon.
 60. The device of claim 56 wherein said first gate electrode isformed over said semiconductor layer with an insulating film interposedtherebetween.
 61. A thin film transistor comprising: a substrate havingan insulating surface; a semiconductor layer formed over said insulatingsurface; a source region and a drain region formed in said semiconductorlayer; a channel region in said semiconductor layer between said sourceregion and said drain region; a first gate electrode adjacent to oneside of said channel region; and a second gate electrode adjacent to theother side of said channel region opposed to said gate electrode,wherein said second gate electrode is overlapped with said source regionand said second gate electrode is not overlapped with said drain region,wherein said second gate electrode is overlapped with a first portion ofsaid channel region so that a second portion of said channel region isoffset from said second gate electrode while said first gate electrodeis overlapped with an entire portion of said channel region, and whereinsaid conductive layer and source region are electrically connected. 62.The device of claim 61 wherein said second gate electrode comprises amaterial selected from the group consisting of a doped polysilicon,chromium, tantalum and tungsten.
 63. The device of claim 61 wherein saidsubstrate is a quartz.
 64. The device of claim 61 wherein said channelregion is formed of crystalline silicon.
 65. The device of claim 61wherein said first gate electrode is formed over said semiconductorlayer with an insulating film interposed therebetween.